Linear and planar electric motors are used in a variety of devices and systems. For example, linear and planar electric motors are used to precisely position a semiconductor wafer during photolithography and other semiconductor processing. The accurate positioning of the wafer during processing is critical to creating high density semiconductor wafers. Alternately, linear or planar motors are used in other devices, including elevators, electric razors, machine tools, metal cutting machines, inspection machines and disk drives.
A linear electric motor generally has a one-dimensional magnet assembly and a one-dimensional coil assembly positioned along the magnet assembly. A planar electric motor generally has a two-dimensional magnet assembly and a two-dimensional coil assembly positioned near the magnet assembly. Electric current in the coil assembly creates a force between the coil assembly and the magnet assembly that can be used to move one of the assemblies relative to the other assembly.
The coil assembly for a linear or planar motor typically includes a coil base having a generally planar, upper surface and a plurality of spaced apart, coils directly secured to the upper surface of the coil base with an adhesive. Mounting the coils directly to the coil base provides a good mechanical connection between the coils and the coil base. This enhances motor controllability and reduces vibration.
Unfortunately, existing linear or planar electric motors are not entirely satisfactory. In particular, electrical current in the coils produces heat due to resistance in the coils. The coils have limited thermal conductivity. During operation, the coils are at a higher temperature than the surrounding environment. Thus, during operation, heat from the coils is transferred to the surrounding environment, including the air surrounding the electric motor and the other components positioned near the electric motor. The heat changes the index of refraction of the surrounding air. This reduces the accuracy of an interferometer system used to monitor the position of the motor and degrades machine positioning accuracy.
As a result thereof, in some applications, including photolithography, it is necessary to accurately control the temperature of the external surfaces of the motor. With the coils glued directly to the coil base, heat from the coils is transferred directly to the upper surface of the coil base and the parts attached to the coil base. Thus, cooling must be provided to both the coil and the side of the coil base that is opposite the coils in order to control the temperature of the external surfaces of the motor.
Additionally, the direct attachment of the coils to the upper surface of the coil base can create a thermal distortion of the coil assembly. In particular, the coils typically are made of a material having a higher coefficient of thermal expansion than the coil base. Further, during operation, the coils are at a higher temperature than the coil base. As a result thereof, the coils expand relative to the coil base. The differential in expansion creates thermal stress in the coils and the coil base and can cause the coil assembly to bend and flex. This reduces the accuracy of movement of the motor.
In light of the above, it is an object of the present invention to provide a device and method for mounting coils in an electric motor that provides a rigid mechanical connection between the coils and the rest of the motor and good thermal isolation of the coils from the rest of the motor. Yet another object of the present invention is to provide a device and method for mounting coils in an electric motor that allows the coils to expand and minimizes thermal distortion. It is another object of the present invention to provide a cooling system to maintain the external surfaces of the motor at a set temperature.